Lord of the RING: ubiquitination and directed degradation of skeletal muscle in acute lung injury

Pseudomonas aeruginosa Quorum Sensing Molecule Alters Skeletal Muscle Protein Homeostasis by Perturbing the Antioxidant Defense System

Skeletal muscle function is compromised in many illnesses, including chronic infections. The Pseudomonas aeruginosa quorum sensing (QS) signal, 2-amino acetophenone (2-AA), is produced during acute and chronic infections and excreted in human tissues, including the lungs of cystic fibrosis patients. We have shown that 2-AA facilitates pathogen persistence, likely via its ability to promote the formation of bacterial persister cells, and that it acts as an interkingdom immunomodulatory signal that epigenetically reprograms innate immune functions. Moreover, 2-AA compromises muscle contractility and impacts the expression of genes involved in reactive oxygen species (ROS) homeostasis in skeletal muscle and in mitochondrial functions. Here, we elucidate the molecular mechanisms of 2-AA's impairment of skeletal muscle function and ROS homeostasis. Murine in vivo and differentiated C2C12 myotube cell studies showed that 2-AA promotes ROS generation in skeletal muscle via the modulation of xanthine oxidase (XO) activity, NAD(P)H oxidase2 (NOX2) protein level, and the activity of antioxidant enzymes. ROS accumulation triggers the activity of AMP-activated protein kinase (AMPK), likely upstream of the observed locations of induction of ubiquitin ligases Muscle RING Finger 1 (MuRF1) and Muscle Atrophy F-box (MAFbx), and induces autophagy-related proteins. The protein-level perturbation in skeletal muscle of silent mating type information regulation 2 homolog 1 (SIRT1), peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1), and uncoupling protein 3 (UCP3) is rescued by the antioxidant N-acetyl-l-cysteine (NAC). Together, these results unveil a novel form of action of a QS bacterial molecule and provide molecular insights into the 2-AA-mediated skeletal muscle dysfunction caused by P. aeruginosa IMPORTANCE Pseudomonas aeruginosa, a bacterium that is resistant to treatment, causes serious acute, persistent, and relapsing infections in humans. There is increasing evidence that bacterial excreted small molecules play a critical role during infection. We have shown that a quorum sensing (QS)-regulated excreted small molecule, 2-AA, which is abundantly produced by P. aeruginosa, promotes persistent infections, dampens host inflammation, and triggers mitochondrial dysfunction in skeletal muscle. QS is a cell-to-cell communication system utilized by bacteria to promote collective behaviors. The significance of our study in identifying a mechanism that leads to skeletal muscle dysfunction, via the action of a QS molecule, is that it may open new avenues in the control of muscle loss as a result of infection and sepsis. Given that QS is a common characteristic of prokaryotes, it is possible that 2-AA-like molecules promoting similar effects may exist in other pathogens.

Novel concepts of acute lung injury and alveolar-capillary barrier dysfunction

In this review we summarize recent major advances in our understanding on the molecular mechanisms, mediators, and biomarkers of acute lung injury (ALI) and alveolar-capillary barrier dysfunction, highlighting the role of immune cells, inflammatory and noninflammatory signaling events, mechanical noxae, and the affected cellular and molecular entities and functions. Furthermore, we address novel aspects of resolution and repair of ALI, as well as putative candidates for treatment of ALI, including pharmacological and cellular therapeutic means.